The invention relates to pressure rolls which can be used in calenders, rolling mills, pulleys for running webs of paper, textile material, metallic or plastic foil, sheets or webs of steel, aluminum or other metals, in papermaking machines and for analogous purposes.
U.S. Pat. No. 3,131,625 to Kusters et al. discloses a roll for the pressure treatment of paper and other web material wherein a hollow cylindrical shell is rotatable about and is movable radially relative to an elongated carrier which is non-rotatably mounted in the frame of a paper making or paper processing machine. Selected portions of the shell can be deformed (so as to regulate the profile of the nip between the roll and an adjacent roll) by hydrostatic bearing elements each of which extends in the circumferential direction of the shell along an arc exceeding 90 degrees. The bearing elements form a row which extends in parallelism with the axis of the shell. The means for biasing the convex external surfaces of the bearing elements against the adjacent portions of the internal surface of the shell comprises cylinder and piston units which are connected or connectable to a source of pressurized hydraulic fluid. The pistons of such units are reciprocable in cylinder chambers which are machined into the carrier, and each piston has a relatively small projection which is received in a socket at the inner side of the respective bearing element. The convex surface of each bearing element is formed with a large number of small pockets and with a centrally located large pocket which receives pressurized hydraulic fluid by way of a bore in the respective piston and its projection. The shell of the patented roll is relatively thin so that it can be deformed by selected bearing elements.
A drawback of the patented roll is that the projections of the pistons act only upon the central portions of the respective hydrostatic bearing elements, i.e., the major portion of each bearing element is ineffective as far as the regulation of the shape of the adjacent portion of the shell is concerned. Moreover, the peripheral portions of the bearing elements are relatively thin and readily deformable so that they cannot offer a pronounced resistance to undesirable deformation of adjacent portions of the shell and are actually likely to be deformed by the shell. Therefore, the patented roll cannot regulate, with a relatively high degree of precision, the width of the nip or nips which is or are defined with the neighboring roll or rolls.
German Auslegeschrift No. 22 30 139 of Spillmann et al. discloses a pressure roll wherein the hydrostatic bearing elements extend beyond both sides of the plane including the axis of the shell and halving the nip. However, the pistons of the cylinder and piston units which are used to urge the bearing elements against the internal surface of the shell are relatively small and act only upon the central portions of the respective bearing elements. This exhibits the aforediscussed drawbacks as concerns the ability of the bearing elements to control the configuration of adjacent portions of the shell. The Auslegeschrift proposes to employ bearing elements having a circular outline or bearing elements having a rectangular outline and extending in parallelism with the axis of the shell.
The just described conventional rolls share the drawback that the magnitude of the load which can be taken up per unit length of the shell is rather limited and is determined by the available or permissible maximum pressure of the hydraulic fluid. If the bearing elements have a circular outline, they can extend well beyond both sides of the plane which includes the axes of the shell and the adjacent roll (namely of the roll which cooperates with the shell to define a nip). However, the number of discrete bearing elements must be reduced proportionally with an increase in their diameter so that the shape of the shell cannot be regulated with a high degree of accuracy. This will be readily appreciated by bearing in mind that the length of those portions of the shell which can be adjusted independently of the other portions (as seen in the axial direction of the shell) will increase proportionally with the number of bearing elements which form a row extending in parallelism with the axis of the roll.
The drawbacks of elongated rectangular bearing elements which extend in parallelism with the axis of the shell are even more serious. Thus, if the bearing elements are relatively long, they cannot bring about an accurate adjustment of relatively short portions of the shell if they can permit any adjustments at all. All such elongated bearing elements can do is to jointly shift the entire shell radially of its carrier in order to widen or narrow the nip which is defined with the neighboring roll. Furthermore, such elongated bearing elements cannot be used in conjunction with relatively thin shells because they are likely to permanently and excessively deform the shell in response to admission of pressurized hydraulic fluid into the respective cylinder and piston units. The shell is likely to develop pronounced facets which extend in the longitudinal direction of the carrier and define ridges which affect the quality and/or integrity of the conveyed material.
U.S. Pat. No. 3,587,152 to Held discloses a pressure roll wherein the supporting device for the shell comprises two rows of hydrostatic bearing elements disposed at opposite sides of and spaced apart from the plane including the axes of cooperating rolls. Reference may be had to FIG. 2 of the patent which shows that the two rows of bearing elements are disposed substantially at the 11/2 and 101/2 o'clock positions of the shell and are remote from the plane including the axes of the shell and of the roll above the shell. Thus, the two rows of bearing elements cannot adequately support the shell at the most important location, namely immediately adjacent the nip.
European patent application No. 0 115 790 of Lehmann proposes to use a third row of hydrostatic bearing elements between two neighboring rows and adjacent the nip (reference may be had to FIG. 5 of this publication). It has been found that such proposal is unsatisfactory because the outer rows of bearing elements can cause excessive deformation of the shell at locations which are spaced apart from the plane including the axes of the rolls which define the nip.
German Offenlegungsschrift No. 30 22 491 (which corresponds to commonly owned U.S. Pat. No. 4,394,793 to Pav et al.) discloses a pressure roll with a row of substantially oval or elliptical hydrostatic bearing elements (see FIG. 3) which are relatively short in the axial direction of the shell but extend to both sides of and well beyond the plane including the axes of neighboring rolls which define a nip. Each such bearing element is acted upon by two cylinder and piston units which are disposed at opposite sides of the aforementioned plane. This eliminates many problems which arise in connection with the utilization of other prior art bearing elements. Each bearing element of Pav et al. can be said to constitute an elongated bridge which extends in the circumferential direction of the shell and overlies the respective pistons. Each bridge can take up and/or transmit large forces and, being relatively short in the axial direction of the shell, each bridge can be used to effect a highly accurate adjustment of the shape and/or position of the respective portion of the shell. It has been found that the roll of Pav et al. can employ a relatively thin (i.e., readily deformable) shell which contributes to accuracy with which the nip can be selected and maintained when the roll is in use. Neighboring bearing elements have parallel facets which extend in the circumferential direction of the shell; this renders it possible to place such neighboring bearing elements close to each other and to thus allow for accurate adjustment of the positions of a large number of shell portions relative to each other and relative to the neighboring roll. However, the need for the provision of several cylinder and piston units for each bearing element contributes to complexity of the supporting device and of the entire roll as well as to complexity of the controls which regulate the admission of hydraulic fluid into and evacuation of hydraulic fluid from the cylinder chambers for the pistons